The purpose of this work is to define the cellular and molecular basis for the steps involved in the process of cell transformation by chemical carcinogens and radiation. Cell mutants of the Balb/3T3 cell line showing different susceptibility to chemically- or ultraviolet (UV)-induced transformation have been isolated in our laboratory. Characterization of these mutants indicates that they are different in the expression process of transformation. The resistant variant showed transformation only when the cells treated with carcinogen were exposed to tumor promoters. Measurement of intercellular communication in the variant cells treated or untreated with tumor promoters suggests that these variants differ in the steps involved in the expression or promotion of transformation, and that inhibition of intercellular communication plays an important role in the promotion of transformation.